1. Field of the Invention
This invention relates to a process, particularly an extrusion coating process, for the production of a laminate comprising a substrate and a polymer coating, particularly a thin polymer coating. In particular, the invention further relates to a process for the production of laminate products useful in medical, apparel, hygiene, agricultural and construction applications, as for example, in a garment, a diaper, or a roof underliner.
2. Description of the Related Art
Processes for the extrusion melt coating of a polymer resin, such as a polyurethane, polyamide or polyester resin, onto non-woven or other substrates are well known. The process generally involves the steps of heating the polymer to a temperature above its melting point, extruding it through a flat die onto a substrate which passes through the curtain of molten polymer, subjecting the coated substrate to pressure to effect adhesion, and then cooling. The extrusion melt coating method is widely used since it allows economical production of a laminated structure in a one-step procedure.
In some cases, the polymer resin layer is capable of forming a bond to the substrate without the requirement of additional adhesive or primer between the substrate and the polymer resin layer. In other cases, adequate adhesion is obtained only by the use of additional adhesive or primer applied to the substrate. Alternatively, a xe2x80x9ctie layerxe2x80x9d is co-extruded with the polymer coating as a compatabilizer in order to adhere the polymer coating to the substrate.
There remain, however, disadvantages to extrusion melt coating processes. In particular, with certain polymer resin and substrate combinations even the use of additional adhesive or primer or a tie layer may not be sufficient to ensure the formation of a strong bond between the polymer resin and substrate such that the laminate product has a high resistance to delamination. This is especially the case when it is desired to produce a laminate having a thin polymer resin layer.
It is considered that one reason for the poor adhesion of certain incompatible polymer resin and substrate combinations, especially when thin films are required, is that the polymer resin coating may cool too rapidly upon contact with the substrate to allow for sufficient time for it to interact with the surface of the substrate and create strong adhesion. It is considered that, typically, the adhesion between an incompatible polymer resin coating and substrate, e.g. for a polyester coating and a polyethylene substrate, consists predominantly of mechanical bonding with little or no chemical bonding. There must generally be sufficiently high penetration of the polymer resin coating into the structure of the substrate to ensure a good bond.
In addition, conventional extrusion melt processes may not be suitable for the production of products which require a thin polymer coating. As noted above, the polymer resin coating may cool too rapidly upon contact with the substrate and this may cause the polymer coating to solidify before forming a layer of consistent thickness.
A further disadvantage of extrusion melt coating processes is that there is a tendency for the formation of pinholes in the polymeric layer. It is important to prevent pinholes and provide a continuous coating layer, for instance to ensure that the laminate structure is waterproof. Pinholing arises since the substrate generally consists of a coarse or porous material. During extrusion coating and subsequent pressing, the molten thermoplastic resin enters the pores or interstices of the substrate and, as a result, the thermoplastic resin film may become disrupted by undulations or fibrous projections on the surface of the substrate. Pinholing is a particular problem in the production of thin polymer resin coatings, and to avoid pinholing in such coatings it is generally required to obtain a low penetration of the polymer resin into the substrate. It is therefore a problem to obtain a pinhole-free thin polymer coating which is strongly adhered to the substrate.
One way of minimizing the problems of poor adhesion and pinholing is to increase the thickness of the polymer resin layer. It is considered that a thicker resin layer has the effect of maintaining the temperature at the interface of the polymer resin and the substrate, which would allow a stronger bond to be formed. In addition, a thicker resin layer would be less susceptible to disruption by irregularities in the substrate and therefore be less susceptible to pinholing.
However, increasing the thickness of the polymer resin layer is economically disadvantageous and is not always appropriate for the end-use of the product. As noted above, it is sometimes desirable that the laminate product comprise a thin polymer film layer. For example, in the case of moisture vapor permeable laminates the additional thickness reduces the moisture vapor transmission rate. Laminate structures are also used as waterproof materials or moisture vapor permeable membranes in the production of, for example, wrapping materials, fabrics, medical materials, packaging materials and the like. As the thickness of the resin layer in such laminates is increased in order to minimize pinholing and provide adequate waterproofing, the desirable characteristics of the non-woven fabric are lost. Laminates suitable for such uses may consist of, for example, a polyethylene resin coating on a non-woven fabric substrate. The thickness of the resin layer in such laminates must be at least 40 xcexcm and preferably 60 xcexcm or more in order to prevent pinholing and this has the effect of making the laminate structure stiff and hard, thereby reducing the value of the product.
EP-A-0611037 discloses a process for making a laminate usable in protective clothing, diapers, and roof underliners. In the process, a moisture vapor permeable, liquid impermeable, barrier layer with a thickness of 3 to 25 xcexcm is coextruded with a 1 to 5 xcexcm thick release layer on one side of the barrier layer and a 1 to 5 xcexcm thick tie layer on the opposite side of the barrier layer. The tie layer is adhered to a porous substrate such as a woven or nonwoven fabric. The tie layer typically comprises a thermoplastic such as an ethylene copolymer or a polyurethane and serves to improve the adherence between the porous substrate and the breathable thermoplastic barrier layer.
EP-A-0295694 discloses an extrusion melt coating process for the production of a waterproof water-vapor permeable laminate, which addresses the problem of combining adequate water-proofing without pinholing while maintaining the desirable characteristics of the substrate material. The thermoplastic polymer resin used to prepare the laminate is required to have a melt viscosity of at least 5000 Pa.s at a temperature 20 to 30xc2x0 C. below the extrusion temperature, and this allows the production of a thermoplastic resin film having a thickness of between 5 and 30 xcexcm. The use of resins which do not satisfy this viscosity requirement is reported as resulting in pinholing. The extruder heating temperature is set such that the melt viscosity of the resin immediately after it is extruded from the die is in the range of 100 to 1000 Pa.s. A resin conforming to this highly temperature-dependent viscosity profile is reported as being relatively unstable in an extrusion process. Accordingly, in the manufacture of the laminate structures of EP-A-0295694 an additional xe2x80x9crelease layerxe2x80x9d (typically polyethylene or polypropylene) having peelability with the first thermoplastic resin layer is co-extruded with the first thermoplastic resin. The release layer is then peeled after cooling to obtain the desired structure.
JP-A-1071742 discloses a laminate for use as a medical waterproof sheet, surgery garment fabric or a wind-breaker fabric comprising a porous substrate and a thermoplastic resin layer having a thickness of between 5 and 30 xcexcm, wherein the thermoplastic resin has a melt viscosity of at least 50000 poise at a temperature 20xc2x0 C. below the extrusion temperature. The process for the production of the laminate also involves coextrusion of the thermoplastic resin layer with an additional release layer.
This prior art is concerned with laminates produced using polymers having a certain minimum viscosity and does not address the problems encountered when it is required to produce very thin films on a substrate utilizing polymers of lower viscosity. Typically, when the viscosity is below a certain level, the molten polymer will more readily flow into the interstices and pores of the substrate which, when the polymer coating is a thin layer coating, will increase the likelihood of pinholing, especially where good bond strength is required between the thin polymer layer and the substrate. These prior art processes also do not address the problem of improving adhesion between incompatible polymer resin and substrate layers. There remains a need for an economical process for producing a laminate structure which has good adhesion between a very thin polymer resin layer and a substrate layer.
A principal object of the invention is the provision of an improved process for the manufacture of a laminate having good adhesion between the polymer resin and substrate layers, particularly a laminate comprising a polymer resin of low viscosity, and particularly a laminate comprising a polymer resin and substrate combination which would not strongly adhere using conventional lamination or extrusion coating processes. A further object of the invention is the provision of a process for the manufacture of laminates having a thin polymer resin layer bonded, with good bond strength, to a resin layer without the formation of holes in the resin layer. A further object of the invention is the provision of a process for the manufacture of waterproof moisture vapor permeable laminates, particularly laminates having differential permeability.